Confined species in SAPO-34 cages participate in methanol reaction and affect product selectivity as well as leading to the deactivation of the catalyst during the MTO process. In this work, spatial- and time-evolution of the confined species in a fixed bed are investigated by TG and dissolution–extraction experiment. Results indicate that both methanol and olefins lead to the formation of confined methylbenzenes and methylnaphthalenes, which are active intermediates in the MTO process. These intermediates further transformed into phenathrene and pyrene in the methanol reaction section and led to the deactivation of the catalyst. A pseudo-steady state period, during which selectivities of products are relatively stable, is achieved while most of cages are occupied by naphthalene and methylnaphthalenes. Confined species reduce the cage volume for products and reactants and thus affect product selectivities. Empty cages are likely to form relatively large-size products like C₄ and C₅₊ molecules. Cages occupied by large-size methylnaphthalenes tend to form more ethene, less propene and even less C₄₊ products than those occupied by relatively small methylbenzenes. Simultaneously, secondary reactions of olefins, which increase the formation of alkanes and C₄₊ products during the initiation period, are greatly reduced in the pseudo-steady state period since olefins are hindered from entering occupied cages. As a result, ethene and propene selectivities and C₂/C₃ ratio tend to increase while C₄₊ and alkane selectivities decrease with the prolonging of time on stream.

Conversion and coking of ethene, propene, 1-butene and their mixtures on SAPO-34 are investigated on a fixed bed reactor. Results indicate that 1-butene maintains a high conversion (ca. 97%), conversion of propene decreases from 64% to 28%, and ethene shows a very low conversion (ca. 14%). However, the conversion of ethene is improved by externally added or in situ formed propene and butene. This difference in olefin conversion is mainly related to the carbocationization tendency of olefins. The coking is also associated with the reactivity of olefins; moreover, the coking is related to the synergy effect between the diffusivity of olefins and the volumetric restriction of cages. Coke derived from the least diffusible 1-butene tends to have single and double ring structures. Coke derived from propene tends to form two or three fused ring structures. In addition, the most diffusible ethene favors the formation of three- or four-ring aromatics in SAPO-34 cages. Moreover, due to the alkylation across cage windows, well-bridged reticulate structure coke is formed in the SAPO-34 crystal. The effects of the olefins' further reaction on the MTO process are also discussed. It is supposed that the olefins' further reaction is one of the main sources of the coking. To design good performance MTO catalysts and to reveal the MTO mechanism, more attention should be paid to the effects of the olefins' further reactions.

Confined compounds in SAPO-34 cages are important to understand the activation and deactivation mechanisms of the methanol-to-olefin process. In this work, gas chromatography–mass spectrometry (GC-MS) chromatograms of CCl₄-extracted samples of used SAPO-34 were denoised by subtracting signals of air compounds and stationary phase bleeding of the chromatographic column, which enhanced the identification of trace compounds. In addition to the generally noted methyl aromatics, this work also identified alkanes, cycloalkanes, alkyl (ethyl, propyl, and butyl) compounds, partially saturated compounds, and bridged compounds. These novel identified trace compounds favor the evolution route depiction of monocyclic, bicyclic, tricyclic, tetracyclic, and multicore hydrocarbons in the SAPO-34 cage. Confined compounds should grow via step-by-step alkylation, cyclization, and aromatization processes. C₂₊ side chains, especially C₃₊, favor the growth of rings. Alkyldihydroindenes should be key intermediates between monocyclic and bicyclic aromatics. Bridged soluble compounds provide evidence that insoluble coke is formed across cages in the SAPO-34 crystal.